1. Field of the Invention
The present invention relates to an electric power steering device comprising an electric actuator, such as an electric motor or the like, which imparts steering force to steered wheels according to rotational actuation of a steering wheel.
2. Description of Related Art
It has been known for this sort of electric power steering device to include an electric motor which imparts a steering assistance force upon rotational actuation of the steering wheel, and this steering assistance force is adjusted by controlling the electrical current which is supplied to this electric motor. While the battery which is fitted to the vehicle is used as a power supply for this type of electric power steering device, if the amount of electric power consumption has become quite large and the capability of the battery (i.e. the battery voltage) has decreased, then the supply of electrical power to the electric motor which generates the steering assistance force is stopped in order to prevent decrease of the power supply voltage. In this case, the assistance function is not restored until the power supply voltage rises above a predetermined voltage. Moreover, the power supply voltage which is required for again starting the assistance function is set to be quite high, in order to prevent occurrence of the hunting phenomenon in which, when the power supply voltage is restored and the assistance function starts again, the power supply voltage again decreases. Accordingly, the time period over which assistance is interrupted becomes undesirably long.
Furthermore, a system is known in which it is arranged, during decrease of the power supply voltage, to limit the assistance torque provided by the electric motor. For example, with the electric power steering device disclosed in Japanese Patent Application Publication No. JP-A-2005-067414, a function is provided of decreasing the assistance torque, when the power supply voltage decreases, by arranging to multiply the assistance torque by an assistance reduction gain, and to change over this assistance reduction gain according to the power supply voltage.
However, because the proportion of assistance reduction is set in advance in the system described in Japanese Patent Application Publication No. JP-A-2005-067414, if the change of the power supply voltage occurs quickly, it sometimes happens that decrease of voltage cannot be avoided.
In other words, because the device disclosed in Japanese Patent Application Publication No. JP-A-2005-067414 is one in which the assistance torque is simply reduced when the power supply voltage decreases and no control is performed to maintain the power supply voltage greater than or equal to a predetermined voltage, the power supply voltage simply decreases to the level at which operation is stopped, so that the steering assistance function is stopped, which is undesirable.